Atoms and molecules in the air move constantly. 【小题1】 In high-pressure systems, the molecules inside the system move faster than those that surround it. The opposite occurs in low-pressure systems, where the air inside the low is moving more slowly than in the surrounding area.

Air flows around a high-or low-pressure system in much the same way as water swirls (旋涡) around a hole. In the Northern Hemisphere, air that moves into a high-pressure system flows outward in a clockwise pattern. Air flowing upward in a region of low pressure moves counterclockwise. 【小题2】

Air is more dense (密集的) in a region of high pressure, so the air pushes out toward less dense regions. The air mass begins to warm as it goes down, which prevents the formation of clouds. The presence of clouds indicates that rain may be present in the air. 【小题3】 In a region of low pressure, the air rises. As it does, the air mass cools and clouds form from the humidity (湿气) inside the air mass.

【小题4】Once the droplets are heavy enough, they fall to the ground. When temperatures are warm, rain is the result. If temperatures are low? the result is snow. When water droplets fall through relatively warmer or colder air masses on the journey to Earth, the low- pressure system may bring snow or ice.

Rain associated with low-pressure systems holds true in much of the Northern Hemisphere. 【小题5】 So in tropical areas, rain association with highs or lows is less predictable.